Theoretical treatment of noise analysis in boiling water reactors

A theoretical model has been developed for calculating power spectral densities and correlation functions of the fluctuating steam void content and corrensponding neutron flux fluctuations in flow channels of boiling water reactors. This has been achieved by solving the mass and energy balance equations under two-phase flow conditions. An analytical treatment is being presented for the simplifying assumptions of constant power density and slip ratio being unity over the core height. The analytical model treats different cases where noise source is either completely correlated or completely uncorrelated in space and where fluctuations of the coolant can be taken into account or not. The results show, that the cases where noise source is assumed to be completely correlated in space can be considered as unrealistic. A full numerical treatment has been developed by taking into account space-dependent power density distribution and slip ratio over the core height as well as exponentially decreasing cross correlation of the noise source in space. The theoretical results have been compared to experimental ones and good agreement is found. (orig.)